The present invention generally relates to accessories for control valve assemblies and, more particularly, to a flow measurement module and method for measuring the flow rate of a fluid through a flow passage regulated by a control valve assembly.
In the process control industries, such as for example, chemical processes, refineries, food and beverage industries, control valve assemblies are routinely employed to control the flow characteristics of fluids, such as for example, gas, steam, water, chemical compounds, etc. within a process. Process control systems typically monitor process control parameters and coordinate the operation of process control devices, such as control valve assemblies, in an attempt to ensure efficient operation of process control systems. It is often desirable to monitor the flow rate of fluids through pipelines regulated by control valves assemblies. In addition, changes in operating conditions or process operations may require the monitoring of flow rates of fluids in pipelines that previously did not require monitoring.
A typical control valve assembly generally includes a valve assembly, an actuator and a positioner, such as for example, an intelligent positioner, a pneumatic positioner, or an analog electronic positioner. The valve assembly is typically defined by an inlet and an outlet with a flow passage extending between the inlet and the outlet. A throttling element is typically disposed in the flow passage to control the flow rate of the fluid. A valve stem or a valve shaft typically connects the throttling element to an actuator. The actuator generally moves the throttling element incrementally into and incrementally out of the flow passage thereby controlling the flow rate of a fluid flowing through the flow passage. In cases where an intelligent positioner is used, the positioner generally receives a position setpoint from a process control system as an input and responsively controls the operation of the actuator to alter the position of the throttling element within the flow passage. Intelligent positioners often include a feedback linkage coupled to the valve stem or the valve shaft to generate a position feedback signal to indicate the position of the throttling element within the flow passage. The position feedback signal is generally routed back to the process control system via the intelligent positioner.
A number of different prior art flow measurement devices are available for measuring the flow rate of a fluid through a pipeline under the regulation of a valve assembly. Examples of such prior art flow measurement devices include vortex meters, magnetic flow meters, coriolis meters and differential pressure transmitters and orifice plates. However, process control operations are often required to be shut down during the installation of such prior art flow measurement devices into operational process control systems. Halting process control operations often leads to monetary losses associated with reduced product output. In addition, the procedures associated with the installation of such prior art flow meters are often complex, time consuming, and expensive since they typically require the cutting of pipelines and the installation of weld flanges to connect the flow meters within the pipelines.
Another prior art flow measurement device generally consists of an intelligent positioner having flow measurement capability. Typically such a prior art intelligent positioner includes a database or table or equation which correlates throttling element position to a valve sizing coefficient associated with the valve assembly being used. The prior art intelligent positioner uses the position feedback signal, representative of the position of the throttling element, to determine associated valve sizing coefficients. A flow rate is derived based on the determined valve sizing coefficients, the pressures upstream and downstream of the throttling element and other fluid characteristics. However, the installation of an intelligent positioner having flow measurement capabilities into an existing process control system can be a time consuming and relatively expensive procedure. Such an installation process would generally necessitate the shutting down of process control operations, often resulting in the loss of valuable production time. It is also likely that the cost of an intelligent positioner having flow measurement capabilities would be as expensive, if not more expensive, than the operating positioner. In addition, the existing positioner or the entire control valve assembly is often removed in its entirety prior to the installation of the prior art intelligent positioner having flow measurement capabilities, resulting in the waste of a functional process control device.
Thus what is required is a cost effective flow measurement device that can be installed into an existing process control system with relative ease to operate in conjunction with existing process control devices while minimizing or eliminating disruptions in process control operations during the installation process and minimizing the unnecessary disposal of functional process control devices.
In accordance with an aspect of the invention, a flow measurement module is adapted to operate in conjunction with a control valve assembly to measure a flow rate of a fluid though a flow passage regulated by the control valve assembly. The control valve assembly generally includes a valve assembly including a throttling element moveable within the flow passage and an intelligent positioner adapted to sense the approximate position of the throttling element within the flow passage and responsively generate a representative position feedback signal. The flow measurement module includes a first communication port adapted to receive an upstream pressure signal representative of a sensed pressure upstream of the throttling element, a second communication port adapted to receive a downstream pressure signal representative of a sensed pressure downstream of the throttling element and a third communication port adapted to receive the position feedback signal representative of the approximate position of the throttling element within the flow passage. A controller is communicatively coupled to the first, second and third communication ports and is adapted to operate in accordance with a computer program embodied on a computer readable medium. The computer program includes a first routine that directs the controller to determine a valve sizing coefficient based on the received position feedback signal and a second routine that directs the controller to issue a flow rate signal representative of the flow rate of the fluid flowing through the flow passage based on the determined valve sizing coefficient, the received upstream pressure signal and the received downstream pressure signal.
In accordance with another aspect of the invention, a method is provided for measuring a flow rate of a fluid though a flow passage regulated by a control valve assembly. The control valve assembly generally includes a valve assembly including a throttling element moveable within the flow passage and an intelligent positioner adapted to sense the approximate position of the throttling element within the flow passage and responsively generate a representative position feedback signal. The method includes the steps of providing a flow measurement module including a controller communicatively coupled to a memory and to first, second and third communication ports. The flow measurement module is communicatively coupled to the intelligent positioner via the first communication port and receives the position feedback signal representative of the approximate position of the throttling element within the flow passage via the first communication port. The flow measurement module receives an upstream pressure signal representative of the sensed pressure upstream of the throttling element via the second communication port and a downstream pressure signal representative of the sensed pressure downstream of the throttling element via the third communication port. A valve sizing coefficient is determined based on the approximate position of the throttling element as indicated by the received position feedback signal. The flow rate of the fluid flowing through the flow passage is determined based on the sensed upstream pressure, the sensed downstream pressure and the determined valve sizing coefficient.
In accordance with yet another aspect of the invention, a method is provided for measuring the flow rate of a fluid flowing through a flow passage within a control valve assembly. The method includes the steps of providing a control valve assembly including a throttling element moveable within the flow passage and Em intelligent positioner adapted to sense a position of the throttling element within the flow passage and providing a flow measurement module. The flow measurement module is communicatively coupled to the intelligent positioner without disrupting the flow of fluid through the flow passage. A flow rate signal representative of the flow rate of the fluid flowing through the flow passage is generated based on sensed pressures upstream and downstream of the throttling element and based on the sensed position of the throttling element within the fluid flow passage.